Under American rules a fumble may be confused with a muff. A muff occurs where a player drops a ball that he does not have possession of, such as while attempting to catch a lateral pass or improperly fielding a kicking play such as a punt (you cannot "fumble" a loose ball). Ball security is the ability of a player to maintain control over the football during play and thus avoid a fumble. Cousins never had possession of the lateral pass, that's why the play continued.